Described herein are an apparatus and a method for providing an inerting gas during soldering. More specifically, described herein are an apparatus and a method for providing an inerting gas during wave soldering using nitrogen and/or other inerting gas.
Work pieces such as printed wiring boards or circuit boards have increasingly smaller wettable surfaces that need to be solder coated and joined. Typical operations for wave soldering involve a soldering bath through which the printed circuit boards or work pieces to be soldered are transported. A conventional automatic wave soldering apparatus includes a flux application, a preheater, and a solder station that is arranged to process printed circuit boards. The printed circuit boards are transported along a moving track or conveyor with their side edges supported by gripping fingers. Flux may be applied by contacting the board with either a foam or spray of flux. The circuit board is then passed through a pre-heating area in order for the flux to reduce the oxides on the metal surfaces to be soldered. The circuit board is then contacted with single or multiple waves of molten solder in an air or inerting gas atmosphere.
The inerting gas atmosphere typically is nitrogen (N2) and/or other inerting gases and is oftentimes called N2 inerting. Soldering within an inert gas and/or nitrogen atmosphere minimizes the formation of dross or oxides on the surface of the solder. The presence of dross and/or an oxide layer is known to cause skips, bridges, or other defects in solder joints. Proximal to the solder waves—which are produced by the wave soldering apparatus during operation—are porous pipes or tubes which run parallel to the solder wave and are used to transport the inerting gas and/or N2 gas to provide a relatively low oxygen atmosphere, particularly underneath the work piece to be soldered.
For lead-free wave soldering, the value of an inerting gas atmosphere comprising N2 is further increased due to the following reasons. The process temperature using common lead-free solders is significantly higher than that of conventional tin-lead solder due to the increased melting points of commonly used lead-free solders. This increase in process temperature promotes dross formation. Furthermore, the cost of lead-free solder is normally much higher than that of conventional tin-lead solder, and the economy loss associated with solder waste by dross formation is more significant than that of lead-free wave soldering. In addition, the wetting performance of lead-free solder is intrinsically poor compared with that of conventional tin-lead solder. Therefore, the quality of the formed solder joints is more sensitive to the state of oxidation on a lead-free solder surface.
It is well known that inerting in wave soldering can significantly reduce dross formation on the molten solder surface. Reducing dross formation not only saves solder material and lessens maintenance requirements, but also improves solder wetting and ensures the quality of the formed solder joints. To apply an inerting atmosphere in an existing wave soldering machine, one common approach is to insert a cage-like protective housing with diffusers mounted inside into the molten solder reservoir. An inerting gas blanket across the solder reservoir can thus be formed, reducing the tendency of solder oxidation.
The diffusers are commonly made of porous tubes that introduce an inerting gas such as N2 and/or other inerting gases into the soldering station. The porous tubes, however, become easily clogged by solder splashing or flux vapor condensation during the wave soldering process. Once the diffuser tube is clogged, the efficiency of inerting will be largely reduced. Present methods of cleaning the diffuser tubes such as, for example, using ultrasonic baths filled with cleaning solutions, are extremely difficult and time consuming. The cleaning of these tubes must be performed on a regular basis and can cause physical damage to the tubes. To avoid these issues, the diffuser tubes are typically replaced once they become clogged rather than cleaned. This increases the overall cost to the end-user.
Accordingly, in order to promote the application of inerting by N2 and/or other inerting gases in wave soldering, it is desirable that the apparatus, method, or both fulfill at least one or more of the following objectives. First, it is desirable that the inerting apparatus and method reduces N2 or other inerting gas consumption to a level such as, but not limited to, 12 cubic meters per hour (m3/hr) or less for inerting a production-scale solder reservoir to meet the cost benefits of applying the technology. Second, it is desirable that the inerting apparatus and method reduces the concentration of O2 above the molten solder surface to a level such as, but not limited to, 2500 parts per million (ppm) or less, or 2000 ppm or less, which corresponds to the cases in which no circuit board is loaded above the solder pot. Third, it is desirable that the inerting apparatus and method uses an apparatus that is simple to install and maintain to minimize retrofitting cost. Moreover, it is desirable that the apparatus or method reduces or eliminates the clogging of the porous diffuser tube to ensure a stable and long lasting inerting performance.